We propose to continue our studies on the spatial organization of the major cell surface phospholipids in eukaryotic cells, with particular emphasis on both their lateral and transbilayer distribution, and on the metabolism of the lipid constituents at the cell surface. The fate of fluorescent and isotopically labeled lipids inserted into the plasma membrane bilayer via fusion or exchange of lipid components from small, artificially generated unilamellar vesicles (liposomes) with the cell surface will be studied. We will examine the kinetics and sites of internalization of these molecules into the cell interior, and will also attempt to look at the synthesis and insertion of newly formed lipids into the cell plasma membrane using appropriate precursors for lipid biosynthesis which are fluorescently labeled. Our studies on the process of membrane fusion using vesicle-vesicle and vesicle-cell systems will be continued. Specifically, we will examine the topology of the fusion process and will attempt to maximize fusion of vesicles with cells.